Method of etching a semiconductor wafer to provide tapered dice



July 12, 1966 Original Filed Feb. 17. 1961 BREAKDOWN VOLTAGE, V ,INVOLTS.

O. M. CLARK METHOD OF ETCHIN A SEMICONDUCTOR WAFER TO PROVIDE TAPEREDDICE 2 Sheets-Sheet l Fig. 3

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HIGHLY ACUTE SURFACE ANGLE NEAR NORMAL SURFACE ANGLE I I I I I I I I IIO 20 3O 4O 5O 6O 70 8090 I00 RESISTIVITY IN OHM-CM BREAKDOWN VOLTAGE ASA FUNCTION OF RESISTIVITY OF N TYPE SILICON COMPARING JUNCTIONS HAVINGNEAR NORMAL SURFACE ANGLES WITH JUNCTIONS HAVING HIGHLY ACUTE SURFACEANGLES.

Fig.2

I I I I I I ATT'YS.

METHOD OF ETGHIN A SEMICONDUCTOR WAFER TO PROVIDE TAPERED DICE 2Sheets-Sheet 2 Original Filed Feb. 17, 1961 I? I8 I9 30 I, I I I I I l II I E/'///// /M// SK FER SIDE ME ADDED W PLA JUNCTI SIDE AY OM JUNCTIONON LASS PAD E. OF FER ON OLTEN WITH WAX CIRCLES. WAX ON PADDLE.

RINSE RINSE IMMERSE PADDLE IN IN HIGH 8 WAFER IN ACID 4 SOLVENT. PURITYWATER. 8 ETCH THROUGH.

REMOVE DICE AND STORE.

Fig. 6

INVENTOR. 0. Melville Clark ATT'YS.

United States Patent 3 260,634 METHOD OF ETCI-IiNG A SEMICONDUCTOR WAFERT0 PROVHDE TAPERED DICE Oscar Melville Clark, Scottsdale, Ariz.,assignor to Motorola, Inc, Franklin Park, 111., a corporation ofIllinois Continuation of application Ser. No. 90,026, Feb. 17,

1961. This application Dec. 22, 1964, Ser. No. 423,633 1 Claim. (Cl.156-17) This application is a continuous of application Serial No.90,026, filed February 17, 1961, now abandoned.

This invention relates to a particular construction for semiconductordice which are the fundamental electrical elements in semiconductorrectifier devices. In particular, the invention relates to a diffusedjunction type rectifying element of a construction and configurationwhich enhances the voltage breakdown characteristics of the element, andto a method of fabricating such a rectifying element.

At the time of filing the above-noted original application there was aneed for a small, lightweight, rugged component which could perform ahigh voltage rectification function in electronic equipment. Thestructure fabricated in accordance with the present invention has filledthat need. Examples of special purpose equipments in which suchcomponents are useful are compact military type transmitters, portabletelevision sets, photoflash units and scintillation counters. Such ahigh voltage element can be used in any equipment where there is arequirement to rectify alternating voltages of over 1000 volts, such asradar equipment, X-ray equipment and static electricity type pollen anddust precipitators.

In order to provide the high voltage breakdowns required for a componentsuch as this prior to the present invention, it was necessary generallyto use in the component a number of semiconductor dice connected inseries, since the over-all high voltage reverse breakdown value of thecomponent is the sum of the breakdown values of the individual dice. Theuse of multiple dice increases costs, complicates the assemblyprocedures and makes it more difiicult to conduct heat from the dieassemblies. Also, the use of multiple dice causes a considerably greateramount of heat to be generated within the package because the heatgenerated is directly proportional to the number of dice used. It isdesirable, therefore, to attain the necessary voltage breakdown valuewith a smaller number of dice in series or with a single die.

An object of the invention is to provide semiconductor rectifyingelements having unusually high reverse breakdown values which can befabricated economically and are suitable for use in mass producedsemiconductor devices.

Another object of the invention is to provide a high voltage rectifyingelement which dissipates a minimum amount of power.

Another object is to provide a high voltage rectifying semiconductordevice which 'has the minimum possible number of die elements therebysimplifying the structure and reducing material and assembly costs.

Another object of the invention is to provide a high voltage rectifyingelement which readily transfers heat generated within it to a heat sink.

A feature of the invention is the attainment of unusually high reversebreakdown voltages in a single semiconductor die element by the use of anovel die configuration in which the peripheral die surface intersectsthe plane of the junction at a highly acute angle which may beconsiderably less than 50, thereby decreasing the voltage gradient whichis produced along the surface of the die at and near the junction in theoperation of the die in a rectifier device. The attainment of very highbreakdown voltages in a single die is of importance because it inicecreases the number of applications in which a component having a singledie can be used, and also because it reduces the number of dice requiredfor applications in which a single die cannot be used.

Another feature of the invention is a method of fabricatingsemiconductor dice having the configuration discussed above by anetching operation which cuts a large number of dice out of a wafer ofsemiconductor material and at the same time inherently shapes the diceso that they have a peripheral surface of changing slope with the leastslope occurring at the junction area, thereby maximizing the voltagebreakdown characteristics of the dice.

Referring now to the drawings:

FIG. 1 is a highly magnified view of the peripheral surface of asemiconductor die element in accordance with the invention showing thehighly acute angle between the plane of the rectifying junctionrepresented by the dotdash line and a plane tangent to the die surfaceat the junction;

FIG. 2 is a graph of breakdown voltage versus resistivity of thesemiconductor material for dice whose surface is nearly perpendicular tothe junction as compared with dice as illustrated in FIG. 1 wherein theangle between the die surface and the rectifying junction is highlyacute;

FIG. 3 is a magnified view of a semiconductor die element showing eachregion of different conductivity and indicating the rectifying junctionin the die and the plated coatings on the upper and lower faces of thedie;

FIG. 4 shows a masked semiconductor wafer mounted in position on apaddle ready for the etching operation which forms the dice of the typeshown in FIGS. 1 and 3;

FIG. 5 is a sectional view of FIG. 4 showing clearly the masked areasand the parts of the water that are removed during the etchingoperation;

FIG. 6 is a process flow diagram showing the processing steps which aredirectly associated with forming the dice; and

FIG. 7 is a sectional view of a typical semiconductor high voltagerectifier showing the location of the semiconductor die element withinthe rectifier housing.

A semiconductor rectifying die in accordance with the invention isshaped somewhat like a truncated cone Whose peripheral surface has avarying slope. An important characteristic of the shape is that at thepoint on the peripheral surface where the rectifying junction isexposed, there is an acute angle between the plane of the junction andthe peripheral surface. It has been found that a rectifying elementhaving this shape exhibits considerably higher reverse breakdown voltagecharacteristics than one in which the junction is more or lessperpendicular to the peripheral surface of the die.

The shaping of the die surface is accomplished in the step of etching awafer so as to divide it into a number of dice by controlling theetching process so that it produces a tapering peripheral surface on thedice. Specifically, the side of the wafer which is closest to therectifying junction is masked with wax or other suitable resist materialwhich completely covers that surface and prevents any etching actionfrom occurring at that surface. The side of the wafer which is fartheraway from the rectifying junction is masked in a pattern such that themasking material covers only those portions of the wafer which will bedice after the etching, and leaves exposed regions between the diceareas which are to be etched away.

The resulting masked wafer is immersed in an etching solution for aperiod of time sufficient for the solution to etch completely throughthe wafer at the exposed regions, thus dividing the wafer into dice. Themain significance of this method is that because the etching die surfaceat nearly a right angle.

action begins at the side of the water away from the junction andprogresses completely through the wafer from that side, the peripheraldie surfaces become increasingly tapered as the etching progresses.Thus, the taper is greatest at the side of thedie closest to thejunction, and this causes the junction to have favorably high breakdownvoltage characteristics.

FIG. 1 is a highly magnified view of the peripheral surface'of arectifier die in according with the invention, and this view clearlyshows the highly acute angle between the plane of the diffusedrectifying junction 2 and a plane 3 tangent to the peripheral diesurface at the point 4 where the diffused junction is exposed.Preferably this angle is less than 50 and satisfactory results have beenobtained-with a surface angle 5 in .the range from about 25 to 30.

FIG. 2 is a plot which clearly shows the advantage of the highly acutesurface angle die configuration shown in FIG. 1 over dice-in whichthe-junction intersects the In this figure the typical breakdown voltagerealized is plotted on the ordihate, and the resistivity of the siliconmaterial is plotted on' the abscissa. The distribution of reversebreakdown voltages for the highly acute surface angle configuration isshown by line 8.in FIG. 2 and this should be compared with thedistribution of breakdown voltages exhibited by the near normal surfaceangle configuration shown as line 9. The various resistivities shownalong the abscissa represent typical resistivities which might beemployed to make a variety of rectifier devices. It can be seen in FIG.2 that at the lower resistivities the ratio of thehighly acute surfaceangle breakdowns to the near normal surface angle breakdowns isconsiderably less than 2 to 1, while in the high resistivity regions of20 ohm-cm. and above, the ratio of the highly acute surface anglebreakdowns to the near normal surface angle breakdowns is 2' to 1 orgreater. The usual resistivi-ties employed for making the high voltagesilicon rectifier devices is in the range of 60' ohm-cm. resistivity andabove. Therefore, the improvement in breakdown voltage provided by thehighly acute surface angle configuration over the near normal surfaceangle configuration is of greatest advantage in higher voltage rectifierdevices. A theoretical explanation in support of this improvement willbe presented in order to provide a fuller understanding ofthe invention.

When a reverse bias is applied to a die 1 as shown in FIG. 1, a spacecharge depletion region is established on each side .ofthe PN junction2. It isan inherent characteristic of PN junctions that the entirereverse voltage is distributed over a relatively small distance oneither side of the junction. The voltage distribution coincides with theregion over which the voltage is distributed. The fact that the entireverse voltage appears across a relatively short distance means that avery great electrostatic field exists over this distance. Experimentshave shown that the breakdown of junctions having configurations of thetype described herein is due largely to surface effects. Since thediffused junction is exposed all around. the die periphery, this exposedregion critically affects breakdown. In situations Where the plane ofthe surface at the junction intersects the plane of the junction at nearnormal surface angles, the field existent along the surface on eitherside of the junction is very high since its location and gradient isdetermined by the space charge region. In dice having the configurationshown in FIGS. 1 and 3, the depletion region area appearing along thedie surface on either side of the junction is stretched. The degree ofstretching out is directly proportional to the reciprocal of thetrigonometric sine of angle 5 of FIG. 1. This spreading out effectresults in a proportionate spreading out of the electrostatic field atthe surface. This reduces the voltage gradient existing along thesurface near the point 4 of FIG. 1. This effeet in turn increases thereverse voltage that can be applied before the junction will break down.

FIG. 3 shows a magnified view of a complete semiconductor die clementsuitable for use in a high voltage rectifier. The central portion of thedie is broken away in this view because of the difficulty of drawingsuch an enlarged view to scale. The basic material 15 of thesemiconductor die element in this instance is silicon. The major upperand lower faces of the die have nickel plated inner layers 11 and 12 andhave outer layers of gold plating 10 and 13. The gold material isreadily solderable, and the nickel material provides satisfactoryadherence of the plated coatings to the silicon. Region 15 is the basicN type silicon material. The diffused junction is located at 2. There isa diffused P+ type region at 14 and a diffused N-|- type region at 16,and both" of these regions are formed by diffusion methods which are notapart of the invention. The P+ region may be formed'by diffusingacceptor impurity material such' as boron into the original N typematerial, and the N+ region'may be formed by diffusing donor typeimpurity material such as phosphorous into the original N type material.The region 16 is of greater electrical conductivity than the N typeregion 15 and is identified therefore as N+. The die-is typically about8 to 12 mils thick andhas a diameter of the order of 70' to mils.

FIG.v 6 is a process flow diagram which presents the important steps ofthe method of forming of the type dice shown in FIGS. 1' and 3. Thismethod will now be presented in detail. A silicon wafer 20 having goldplated faces 10 and 13 (FIG. 4) is clamped within a metallic mask havinga continuous pattern of holes on one side'whose diameter isapproximately one hundred thousandths of an inch. After the wafer isproperly secured within the mask, acid resistant wax is sprayed throughthe holes in the mask causing wax circles 19 to be imprinted on one faceof the wafer. It is very important that the wax circles appear on theface of the wafer farthest away from the PN junction 2, since it isdesired to etch from this side. This method permits the attainment ofthe highly acute surface angle in the diffused junction region aspreviously mentioned. A glass paddle 17 shown in FIG. 4 is placed on ahot plate and additional wax 18 is melted on the surface of the paddle.When this wax 18 is completely melted into a puddle, the wafer 20 isplaced in position on the molten wax with the junction side facing thepaddle. The glass paddle 17 with the Wafer 20 mounted on it is thenremoved from the hot plate and the wax is permitted to solidify. Thissecures the wafer to the paddle 17.

The paddle 17 withthe wafer 20 mounted on it is then immersed in an aquaregia etching solution which removes the gold plating on the exposedwafer face. The paddle and wafer are rinsed in high purity water, andthe assembly is then immersed in a hydrofluoric-mitric-acetic acidsolution which cuts through the silicon material. A typical region whichis etched away is shown at 30 in FIG. 5. This etching continues untilthegold plating 13 becomes visible at the lower wafer face. The assembly isthen rinsed in high purity water and is then immersed again in aquaregia to remove the lower face gold plating 13. The assembly is againrinsed in high purity water and is finally etched in ahydrofluoric-nitric acid solution. Another rinsing is then performed inhigh purity water and finally the assembly is rinsed in anultrasonically agitated solvent bath. This causes the dice to separatefrom the assembly. The dice are then dried and stored.

FIG. 5 shows quite clearly, in a sectional view, the regions 30 on whichthe various acid baths operate to cut through the upper layer of goldplating, the silicon and also the bottom layer of gold plating. Thecondition of the die units shown in FIG. 5 is the condition that existsfollowingthefinal aqua. regia etching and prior to the solvent rinsingstep. Since the etching progresses through the wafer from the side awayfrom the rectifying junction, the etching produces dice with a taperedsurface as shown in FIGS. 1 and 3 wherein the surface angle at thejunction is highly acute.

FIG. 7 is .a sectional view of a complete high voltage rectifier showingthe die 1 located in its usual position mounted on the heat sink 26.This rectifier package is shown merely as one of many suitable packagesfor the die of the invention. The die 1 has been soldered in position onthe heat sink by the solder layer 28 and the dies upper face has beensecured to the S bend lead 29 by means of solder layer 24. The S bendlead 29 extends through, and is welded to the tube 21. The tube 21 is anintegral part of the header 23 and is held in place by the glass region22. The electrical lead 27 is welded to the heat sink 26 and providesthe electrical contact to one side of die 1 through the heat sink andthe soldered layer 28.

The particular rectifier die configuration described above results inunusually high reverse breakdown voltage characteristics as demonstratedby the data plotted in FIG. 2. The importance of the reducedelectrostatic stress at the surface of the die in the region of thejunction has been emphasized. When the angle of intersection between theplane of the junction and a plane tangent to the peripheral surface ofthe die at the junction is quite acute, the depletion layer which isproduced in the electrical operation of the die as a rectifier spreadsalong a longer surface than when the junction is substantiallyperpendicular to the surface. This reduces the voltage gradient alongthe surface and since the effective surface field is thus weakened,there is less tendency for the junction to break down due to adverseeffects of the surface field at the junction. Thus, higher reversevoltages can be applied to the die without causing the junction to breakdown.

The ability to use a single die in a high voltage rectifier is importantbecause the amount of heat generated Within the rectifier package isless when it has a single die than when it has more than one die. Also,heat is readily transferred from a single die to a heat sink. Where morethan one die is required, a smaller number of dice may be used in agiven application and this reduces the adverse effect ofdisproportionate voltage drops which may be produced across multiple dieimmediately after application of a reverse voltage. Also, material costsand assembly costs may be reduced and the assembly operation issimplified. Dice produced by other methods do not result in as favorablean angle of intersection of the die surface with the plane of thejunction and therefore have inherently lower breakdowns. Since thevoltage breakdown characteristics of a die are affected strongly bysurface conditions at the junction, the voltage gradient at the surfaceof the die near the junction is a major contributing factor in thebreakdown phenomenon. Thus, the advantage of the particular dieconfiguration and etching method of the invention can be readily seen.

I claim:

A method of manufacturing a plurality of dice from a semiconductorwafer, each of which dice has a rectifying junction therein and isadapted for assembly in a rectifier, said method including providing ametal coating on each of the two opposite faces of a semiconductorwafer, applying a masking composition on the metal coating on one faceof the wafer which composition outlines thereon a pattern for the outeredge of each of the plurality of dice to define such dice, applying anacid resistant wax coating to and mounting a fiat-faced graspable memberon the metal coating on the other face of the wafer, immersing saidgraspable member and wafer as an assembly in an etching solution andetching away said metal coating and the wafer on said one face in theoutline corresponding to the pattern of the masking composition on saidone face and etch-tapering each defined dice in the wafer at the etchedsurface of each in a configuration wherein the angle of greatest taperfor each dice is at the rectifying junction, which said junction isnearer said other face than it is to said one face of the wafer and'istapered less than an angle of and etch-cutting entirely through saidwafer coincident with said etching-away to provide the plurality of diceaccording to said pattern of the masking composition, and separatingsaid etched-cut dice from the graspable member for subsequent assemblyof each dice in a rectifier.

References Cited by the Examiner UNITED STATES PATENTS 2,672,528 3/ 1954Shockley. 2,879,147 3/1959 Baker 156-11 2,944,321 7/ 1960 Westberg.2,951,191 8/1960 Herzog. 3,046,176 7/ 1962 Bosenberg.

ALEXANDER WYMAN, Primary Examiner.

JACOB STEINBERG, Examiner.

